1. Field of the Invention
The present invention relates to video cassette recorders (VCRs) and, more particularly, to a method and an apparatus for controlling the speed of a capstan motor in a video cassette recorder (VCR).
2. Description of the Related Art
In general, a video cassette recorder (VCR) is a device for recording and reproducing a video signal and an audio signal recorded on a recording medium (i.e., a video cassette tape). The VCR can reproduce the recorded video signal at a normal, low or high speed, or can stop reproducing the signal.
The servo system of the VCR is divided into a drum servo which controls the rotation of a head, and a capstan servo which controls the track and maintains the running speed of the recording medium. Specifically, the capstan servo controls the phase and speed of the capstan motor so that the head which is allotted to each channel can accurately reproduce or playback the video signal of the relevant channel. The capstan servo also regularly maintains the running speed of the recording medium when reproducing or recording. The rotation speed of the capstan motor is controlled by obtaining a capstan speed error voltage from a capstan frequency generating (CFG) signal corresponding to the running speed of the recording medium. In the case of a low-speed reproducing, a temporary stop, a classifying operation, etc., a slow step signal outputted from the microcomputer is applied to the capstan motor.
The phase of the capstan motor is controlled by adding the capstan speed control voltage to a capstan phase error voltage which is obtained by comparing the frequency of a control signal with a reference frequency when reproducing.
In the case of minutely controlling the capstan motor, such as, for example, in low-speed reproducing, it is necessary to control the conventional capstan frequency generating (CFG) signal for a duration equal to one-half the period of the CFG signal. Conventionally, the speed control is performed by simultaneously detecting a high edge and a low edge of the CFG signal. That is, each time interval from a high edge to a low edge or a low edge to a high edge is detected and then compared with a predetermined target time interval, so that the speed of the capstan motor can be controlled based on the difference therebetween.
However, the conventional method for controlling the speed of a capstan motor is not without deficiency. Some of these deficiencies will now be described. First, when the capstan motor rotates at a constant speed, the time periods corresponding to each section of the pulse illustrated in FIG. 1 should be equal to each other (A=B=C=D). However, due to certain mechanical attributes of the capstan motor, the time periods corresponding to the high sections do not equal the time periods corresponding to the low sections (Axe2x89xa0B, Cxe2x89xa0D, A=C and B=D). As a result, as the capstan motor rotates at a continuously varying speed, the capstan motor vibrates, thereby degrading the performance of the capstan servo. Second, due to the vibration of the capstan motor, undesirable noise is generated.
Therefore, it is an object of the present invention to stably control the rotation speed of a capstan motor, even when the time intervals corresponding to a high section and a low section of a capstan frequency generating signal are different.
It is another object of the present invention to provide a method and an apparatus for controlling the speed of a capstan motor in a video cassette recorder (VCR).
According to one aspect of the present invention, a method for controlling the speed of a capstan motor in a video cassette recorder (VCR) includes modulating a capstan frequency generating (CFG) signal into a pulse signal. Time intervals corresponding to one of a rising edge to a next rising edge and a falling edge to a next falling edge of the pulse signal are calculated. The calculated time intervals are compared to a target time to obtain a difference therebetween. A rotation speed of the capstan motor is compensated based on the obtained difference to control the rotation speed of the capstan motor.
According to another aspect of the present invention, an apparatus for controlling the speed of a capstan motor in a video cassette recorder (VCR) includes a capstan motor for transporting a recording medium at a predetermined speed, and generating a capstan frequency generating (CFG) signal. A pulse transformer modulates the CFG signal into a pulse signal. A delay unit generates a delayed pulse signal. A first adder adds the pulse signal and the delayed pulse signal to calculate time intervals corresponding to one of a rising edge to a next rising edge and a falling edge to a next falling edge of the pulse signal. A second adder adds a calculated time interval from the first adder and a target time interval to calculate the difference therebetween. A gain controller outputs a gain control signal to the capstan motor based on the difference calculated by the second adder.
Preferably, the apparatus includes an amplifier operatively coupled to the capstan motor and the pulse transformer for amplifying the CFG signal. Also, preferably the apparatus also includes a filter operatively coupled to the gain controller and the capstan motor for removing noise from the gain control signal.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings, where the same reference numerals are used to represent the same functional elements.